In the case of supplying and storing liquefied gas in a tank for storing liquefied gas, it is necessary to measure, monitor and control the amount of cold liquid. In order to measure liquid level of liquefied gas for that purpose, a mechanical, optical or electrical method is known as a method of measuring respective types of liquid level. However, a mechanical method, which enables detection of changes in the continuous liquid level, has disadvantages that accuracy is not good. In addition, the optical method and the electrical method with resistant elements result in noncontiguous detection to present a problem. Therefore, as the present time, any method is insufficient in performance from the view point of accuracy and detection mode as a sensor device for measuring a cold liquid level.
As a method for solving such a problem, a measuring method with superconductive sensor device beam is known as (see, for example, JP-A-2000-275085). Actually, the superconductive sensor device beam with a NbTi cord practically used as a liquid level sensor device for liquefied gas which measures liquid helium is electrically connected to a current lead for carrying current for measurement in the vicinity of the upper end and the lower end and a voltage lead for measuring voltage. In principle thereof, significant changes in electrical resistance in the normal conductive state at the time of the superconducting transition state of a superconductor is utilized to detect changes in electrical resistance between a liquid level sensor device end portions and to measure thereby height of liquid level since distribution of electrical resistance in a sensor device changes according to temperature distribution appearing between the portion dipped in liquid of a sensor device and the portion of the sensor device exposed on the upper side of liquid level. Such a feature does not need integrating a window portion for viewing a liquid level directly into an apparatus, and is less influenced by frost and the like.
As an important example of application of a liquid level sensor device for liquefied gas, there is a hydrogen station for fuel cells, development of which is progressing rapidly. A fuel cell generates electric power by chemical reaction between hydrogen and oxygen. Emission of the fuel cell is only water and the fuel cell is potential to become a power supply which is extremely friendliness to the global environment. Hydrogen being fuel for fuel cells exists little in the natural world and therefore is mainly produced artificially. Storage is required after production and is considered to be implemented with hydrogen in a liquid state as a representative example of method thereof. In this case, it is necessary to consistently monitor the liquid level of liquid hydrogen.
However, there are problems that when the temperature of liquid hydrogen becomes 20 K of the boiling temperature, it is difficult to use conventional NbTi cords and the high-temperature superconductive wire in the aspect of a critical temperature and accuracy. Therefore, it is desired to develop a liquid level sensor device for liquefied gas with superconductor the critical temperature of which is 21 K to 31 K and which is produced inexpensively. A sensor device with oxide superconductor such as Bi system oxide and the like with higher critical temperature is proposed as well (see JP-A-2002-202175). Production thereof, however, is not necessarily easy and costly burden is significant, resulting in a problem for building a stable sensing system.